Physiological response to posture change

ABSTRACT

In an embodiment, an implantable medical device includes a controller circuit, a posture sensing circuit, and a physiological sensing circuit. The controller circuit senses a change in a physiological signal as a result of a change in posture, and generates a response as a function of that change. In another embodiment, the controller circuit identifies a heart failure condition as a function of the change in the physiological signal.

TECHNICAL FIELD

Various embodiments relate to the field of implantable medical devices, and in an embodiment, but not by way of limitation, to the detection by an implantable medical device of a physiological response to posture changes.

BACKGROUND

A person's health status may be determined by analyzing physiological parameters such as heart rate, respiration pattern, and blood pressure, just to list a few. These physiological parameters may follow different patterns for a healthy individual versus a person who is suffering from some form of ill health. Some physiological parameters are affected by the posture of the body. Additionally, the present inventors have recognized that the response of these parameters to changes in posture may be affected by the person's state of health. Consequently, the present inventors have recognized that medical and health care profession would benefit from a system and method to detect, capture, and analyze the effect of posture change on such physiological parameters.

SUMMARY

In certain examples, an implantable medical device includes a controller circuit, a posture sensing circuit, and a physiological sensing circuit. The controller circuit senses a change in a physiological signal as a result of a change in posture, and can optionally respond to that change. In certain examples, the controller circuit identifies a heart failure condition or status using the change in the physiological signal.

This summary is intended to provide an overview of the subject matter of the present patent application. It is not intended to provide an exclusive or exhaustive explanation of the invention. The detailed description is included to provide further information about the subject matter of the present patent application.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, which are not necessarily drawn to scale, like numerals describe similar components throughout the several views. The drawings illustrate generally, by way of example, but not by way of limitation, various examples discussed in the present document.

FIG. 1 illustrates a block diagram of an example embodiment of an implantable medical device.

FIG. 2 illustrates an example embodiment of a process to determine a physiological response to a change in posture.

FIG. 3 illustrates an example embodiment of an implanted medical device coupled to an adjunct device.

DETAILED DESCRIPTION

The following detailed description refers to the accompanying drawings which form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. These embodiments, which are sometimes referred to as examples, are discussed in sufficient detail to enable those skilled in the art to practice the invention, and such embodiments may be combined, or that other embodiments may be utilized and that structural, logical and electrical changes may be made without departing from the scope of the present invention. The following detailed description provides examples, and the scope of the present invention is defined by the appended claims and their equivalents.

It should be noted that references to “an”, “one”, or “various” embodiments in this disclosure are not necessarily to the same embodiment, and such references contemplate more than one embodiment.

The system and method described herein provide a system and method to detect a physiological response to a posture change. In one or more examples, the physiological responses include one or more of a change in heart rate, a change in heart rate variability, a change in heart sounds, a change in respiration rate, and a change in blood pressure. The change in posture may include one or more of a change from a more upright posture to a more recumbent posture, a change from a more recumbent posture to a more upright posture, and a change within a recumbent posture (e.g., changing from a supine position to a right lateral decubitus position). The upright posture may include a standing posture or sitting posture or both, and the recumbent posture may include one or more of supine, prone, right lateral decubitus, and left lateral decubitus postures.

FIG. 1 illustrates an example of an implantable medical device 100. In this example, the device 100 includes a controller circuit 110. The device 100 further includes a posture sensing circuit 140. The posture sensing circuit 140 may include one or more of a tilt switch, a single axis accelerometer, and a multi-axis accelerometer. The device 100 can include one or more of a memory circuit 130, an energy delivery circuit 112 to deliver energy to a heart, a drug delivery circuit 114, a neural stimulation circuit 116, a physiological parameter sensing circuit 120, a telemetry circuit 150, a comparator circuit 155, and an activity sensing circuit 118—any or all of which can be coupled to the controller circuit 110. The telemetry circuit 150 is generally capable of being wirelessly coupled to an external device 160. The physiological parameter sensing circuit 120 can include one or more of several sub-circuits, such as a heart rate sensing circuit 122, a heart sound sensing circuit 124, a heart rate variability sensing circuit 125, a respiration sensing circuit 126, or a blood pressure sensing circuit 128. The energy delivery circuit 112 may include one or more of a pacing circuit, an anti-tachyarrhythmia pacing (ATP) circuit, a cardiac resynchronization therapy (CRT) circuit, or a defibrillation or cardioversion circuit.

In an example, the device 100 is implanted into a patient. The posture sensing circuit 140 senses the patient's posture, and transmits a resulting posture signal to the controller circuit 110. For example, this posture signal may indicate that the patient is recumbent, seated, standing, prone, supine, and/or in a right or left lateral decubitus position. Substantially concurrently, the physiological signal sensing circuit 120 senses at least one other physiological signal—different from posture, but which may be affected by posture. Examples of such physiological signals may include heart rate, heart sound, heart rate variability, respiration, blood pressure, or the like. The physiological sensing circuit 120 transmits to the controller circuit 110 a physiological signal indicative of the sensed physiological parameter. In response to the received posture and physiological signals, the controller circuit 110 determines whether there is a change in the physiological signal in response to a change in the patient's posture. The controller circuit 110 can further generate a response as a function of the posture and physiological signals, such as in response to a change in the physiological signal resulting from a change in the patient's posture. Such a response can include transmitting a control, alert, or other signal to one or more of the energy delivery circuit 112, the drug delivery circuit 114, the neural stimulation circuit 116, and the telemetry circuit 150. In certain examples, such a response can indicate whether a patient's heart failure condition is improving or getting worse.

In certain examples, the physiological signal is a heart rate, and the controller 110 determines a change in heart rate caused by a change in posture. A normal patient's heart rate typically increases upon standing up. However, while a patient with heart failure will also experience an increase in heart rate upon standing up, such increase is believed to generally be less than the corresponding increase for the normal patient. Thus, in certain examples, the controller 110 can compare the heart rate response to posture to a normal patient's response to determine an indication of whether the patient has heart failure, or whether the patient's heart failure is improving or worsening. The heart failure patient's smaller increase in heart rate upon standing can be used in several different ways. For example, the device may use this information to identify that a patient is experiencing heart failure, to identify whether the heart failure is improving or worsening, or both. For example, if a heart failure patient's heart rate response to a change in posture, such as standing, trends towards a normal subject's response, then the heart failure patient's condition can be deemed to be improving. If the heart failure patient's heart rate response to the posture change trends away from a normal person's response (e.g., the patient's trend is toward an even smaller increase in heart rate upon standing), the heart failure patient's condition can be deemed to be getting worse. In certain examples, in response to the improving, worsening, or constancy of the patient's heart failure condition, the controller circuit 110 can transmit a signal to the energy delivery circuit 112, drug delivery circuit 114, neural stimulation circuit 116, or telemetry circuit 150, such as to initiate or adjust a responsive heart failure or other therapy or to deliver an alert to the patient or a caregiver.

In another example, the blood pressure sensing circuit 128 senses a change in a person's blood pressure resulting from a change in posture. Changes in a normal patient's blood pressure are typically quite small when going from a more recumbent position to a more upright position due to a compensatory baro-reflex response of the body to maintain blood pressure homeostasis. However, a person suffering from heart failure typically experiences a greater decrease in blood pressure than a normal person. Information about the degree of change in blood pressure in response to a posture change (or a comparison of this response to the response of a normal patient) can be used by the controller circuit 110 to determine whether heart failure is present, improving, or worsening. As discussed above, the controller circuit 110 can use this information to identify heart failure or its improvement or worsening, and can optionally transmit a responsive signal to one or more of the energy delivery circuit 112, the drug delivery circuit 114, or the neural stimulation circuit 116, such as to initiate or adjust a responsive therapy, or to the telemetry circuit 150, such as to deliver an alert to the patient or a caregiver.

In another example, the heart sound sensing circuit 124 senses a change in one or more heart sounds caused by a change in the patient's posture. The sensed heart sounds may be the normal heart sounds S1 and S2 caused by the closure of the mitral and tricuspid valves, and the closure of the aortic and pulmonary valves respectively. The sensed heart sounds may also include the abnormal heart sounds S3, S4, and mitral regurgitation. The S3 and S4 heart sounds both relate to ventricular diastolic filling. A normal patient will typically experience an increase in heart rate upon becoming more upright. This also typically results in an increase in heart sound energy. However, a heart failure patient will typically experience a smaller increase in heart rate upon changing to a more upright posture. As a result, the heart failure patient will typically experience a smaller increase in heart sound energy than a normal person. This information (or a comparison of this response to the response of a normal patient) can be used to determine whether heart failure is present, improving, or worsening, or if therapy should be initiated or adjusted in response to the change, or if a responsive alert should be delivered to the patient or a caregiver. While this example indicates that heart sound energy will not increase in a heart failure patient as much as in a normal patient, other effects relating to heart sounds for other types of heart failure conditions or other diseases may manifest themselves.

In another example, the heart rate variability (HRV) sensing circuit 125 senses a change in heart rate variability in response to a change in posture. A normal person's heart rate varies in response to several factors, including a change in posture. However, it is believed that a heart failure patient's heart rate variability will exhibit a smaller change in response to a posture change than a normal person. This information (or a comparison of this response to the response of a normal patient) can be used to determine whether heart failure is present, improving, or worsening, or if therapy should be initiated or adjusted in response to the change, or if a responsive alert should be delivered to the patient or a caregiver. In certain examples, frequency components of HRV may be extracted such as a high frequency (HF) component of HRV and a low frequency (LF) component of HRV. An LF/HF ratio can be used to assess sympathetic/parasympathetic balance of the autonomic nervous system. For a healthy individual, the LF/HF ratio may change by as much as 20% when transitioning a supine to an upright posture. However, it is believed that a heart failure patient will not experience as much of a change in LF/HF ratio in response to such a postural transition since sympathetic levels are already elevated. Similarly, this information (or a comparison of this response to the response of a normal patient) can be used to determine whether heart failure is present, improving, or worsening, or if therapy should be initiated or adjusted in response to the change, or if a responsive alert should be delivered to the patient or a caregiver.

In another example, the respiration rate sensing circuit 126 senses a change in respiration rate in response to a change in posture. A normal person will typically experience an increase in respiration rate upon becoming more upright. By contrast, it is believed that a heart failure patient will typically experience a decrease in respiration rate upon becoming more upright. This results from thoracic fluid accumulation in a recumbent heart failure patient that can make breathing difficult when recumbent, resulting in a compensatory increase in respiration rate when recumbent. Upon becoming more upright, some thoracic fluid moves out of or away from the lungs, thereby making it easier to breathe. This information (or a comparison of this response to the response of a normal patient) can be used to determine whether heart failure is present, improving, or worsening, or if therapy should be initiated or adjusted in response to the change, or if a responsive alert should be delivered to the patient or a caregiver.

As discussed above, the controller circuit 110 may be coupled to one or more of an energy delivery circuit 112, a drug delivery circuit 114, or a neural stimulation circuit 116. When coupled to the energy delivery circuit 112, any pacing or cardiac resynchronization therapy delivered to the patient may be based at least in part on the information received from the posture sensing circuit 140 and the physiological parameter sensing circuit 120. The pacing or cardiac resynchronization therapy can be adjusted many different ways, such as by changing the rate of the delivered pulses, changing the amplitude of the delivered pulses, changing the pulsewidth of the delivered pulses, adjusting the location in the patient's heart where the pacing pulses are delivered, adjusting AV-delay, inter-ventricular delay, intra-ventricular delay, or adjusting anti-tachyarrhythmia therapy.

In certain examples, the controller circuit 110 is coupled to a neural stimulation circuit 116. The controller circuit 110 can cause the neural stimulation circuit 116 to transmit one or more stimulation pulses to the autonomic nervous system (ANS) in response to a postural change in one or more physiological signals, where such stimulation of the ANS is different from issuing a stimulation pulse to capture cardiac tissue to evoke a resulting heart contraction. For example, if the heart rate variability sensing circuit 125 senses a lesser change HRV when the patient changes posture, then one or more responsive neurostimulation pulses delivered to one or more ANS locations to influence the autonomic balance and obtain a more normal HRV response to posture.

In certain examples, the device 100 can include a drug delivery circuit 114 coupled to or included in the controller circuit 110. Using the physiological response to posture information discussed above, a pharmaceutical or other substance may be titrated into the patient. For example, if over time the patient exhibits a smaller heart rate increase upon becoming more upright, a diuretic or one or more pharmaceuticals may be delivered to patient that relieve pulmonary fluid congestion. In certain examples, a cardiac resynchronization therapy may be additionally initiated or adjusted by the energy delivery circuit 112, such as to attempt to increase or restore the heart rate increase upon becoming upright.

In certain examples, the device 100 determines the extent of the postural change in the physiological signal by computing one or more ratios. In other examples, the extent of the postural change may be determined by a difference or other mathematical relationship. In certain examples, the computation of the ration or other mathematical relationship may use a comparator circuit 155 that is coupled to or incorporated in the controller circuit 110. For example, this can include calculating a ratio of a physiological signal in a first posture to the physiological signal in a second posture. This ratio can be computed for a healthy person and stored in the memory circuit 130 or the external device 160. The device 100 calculates a similar ratio for the particular patient in which the device is implanted, such as by using information received from the physiological sensing circuit 120 and the posture sensing circuit 140. These ratios can then be compared at a particular time or over a time period, such as to determine the particular patient's health status, which may include information about improvement or worsening of such health status, particularly with respect to heart failure. In certain examples, the ratio is calculated as: 100*(RPP1/RPP2−1) In the above equation, RPP1 relates to a resting physiological parameter value while in a first posture, and RPP2 relates to a resting physiological parameter while in a second posture. The activity sensing circuit 118 may be used to determine periods when the person is at rest. Examples of physiological parameters that can be used in the above ratio include, by way of example, but not by way of limitation, heart rate, heart rate variability, heart sound amplitude, respiration, blood pressure, or other physiological parameter that is affected by posture. Additionally or alternatively to the above ratio, a difference or other mathematical relationship may also be used to compare the patient physiological response to posture to a threshold value, where the threshold value can be established, in certain examples, using a healthy person's physiological response to posture.

FIG. 2 is a flowchart that illustrates an example of a process 200 to determine, analyze, and respond to a postural change in a physiological parameter. At 210, a change in posture of a body is detected. At 220, a change in a physiological parameter in response to the change in posture is determined. At 230, a degree in the change of the physiological parameter is determined. This can be accomplished by comparison to a threshold value, a comparison to a trend of similar values, or the like. At 240, an identification of a heart failure status (e.g., presence, improvement, or worsening) can be identified. At 250, a therapy, alert, or other response may be generated based on the heart failure status information or directly from the postural change in the physiological parameter.

In certain examples, the device 100 includes a telemetry circuit 150, such as for communicating with an external device 160. In certain examples, the telemetry circuit 150 transmits data collected by the posture sensing circuit 140 and/or the physiological parameter sensing circuit 120, either before or after signal processing. This data may then be analyzed, such as by the external device 160 or by a health care professional to determine the patient's status. Additionally, in certain examples in which the device 100 or the external device 160 determines (e.g., using the postural change in the physiological signal) that the heart failure is worsening (such as during a decompensation episode) a resulting alert may generated and delivered to the patient or a caregiver. As illustrative examples, the alert may include one or more of an audible signal, a text message, or some other signal to draw attention to the patient's worsening condition. This may help avoid hospitalizations resulting from decompensation episodes.

FIG. 3 is a diagram illustrating an example of a medical device system 300 which can be used in connection with transmitting data from an implanted device 100 to an external device 160. FIG. 3 illustrates a body 302 with a heart 305. System 300 includes an implantable medical device 100, a lead system 308, an adjunct device or system 160, and a wireless telemetry link 360. Posture data, physiological parameter data, and other data may be transferred from the device 100 to the external system 160 via the telemetry link 360. The telemetered data loaded into the external system 160 can then be used for analysis and interpretation either immediately or at a later time.

In the foregoing detailed description, various features are grouped together in one or more embodiments or examples for the purpose of streamlining the disclosure. This method of disclosure is not to be interpreted as reflecting an intention that the claimed embodiments of the invention require more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive subject matter lies in less than all features of a single disclosed embodiment. Thus the following claims are hereby incorporated into the detailed description of embodiments of the invention, with each claim standing on its own as a separate embodiment. It is understood that the above description is intended to be illustrative, and not restrictive. It is intended to cover all alternatives, modifications and equivalents as may be included within the scope of the invention as defined in the appended claims. Many other embodiments will be apparent to those of skill in the art upon reviewing the above description. The scope of the invention should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled. In the appended claims, the terms “including” and “in which” are used as the plain-English equivalents of the respective terms “comprising” and “wherein,” respectively. Moreover, the terms “first,” “second,” and “third,” etc., are used merely as labels, and are not intended to impose numerical requirements on their objects.

The abstract is provided to comply with 37 C.F.R. 1.72(b) to allow a reader to quickly ascertain the nature and gist of the technical disclosure. The Abstract is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. 

1. A system comprising: an implantable medical device, the implantable medical device comprising: a posture sensing circuit configured to detect a change within a recumbent posture of a subject; a physiological signal sensing circuit to sense a physiological signal; a controller circuit, operatively coupled to the physiological signal sensing circuit and the posture sensing circuit, the controller circuit configured to detect a change in the physiological signal that occurs in association with the detected change within the recumbent posture of the subject, wherein the physiological signal sensing circuit includes at least one of a heart rate sensing circuit, a blood pressure sensing circuit, a heart sound sensing circuit, a heart rate variability sensing circuit, and a respiration rate sensing circuit, and the change in the physiological signal includes at least one of: a sensed change in heart rate that is less than a normal subject's change in heart rate; a sensed change in blood pressure that is different from a normal subject's change in blood pressure; a sensed change in heart sound energy that is different from a normal subject's change in heart sound energy; a sensed change in heart rate variability that is different from a normal subject's change in heart rate variability; and a sensed change in respiration rate, respectively, and wherein the controller circuit is further configured to generate a response as a function of the detected change in the physiological signal; and a circuit to compare the change in the physiological signal, the comparison comprising a comparison between (1) a ratio of the physiological signal in the first recumbent posture to the physiological signal in the second recumbent posture for a subject with a possible health condition, to (2) the ratio of the physiological signal in the first recumbent posture to the physiological signal in the second recumbent posture for a normal subject or population.
 2. The system of claim 1, wherein the ratio comprises: 100*(RPP1/RPP2−1); wherein RPP1 relates to a resting physiological parameter while in a first posture; and wherein RPP2 relates to a resting physiological parameter while in a second posture.
 3. A system comprising: an implantable medical device, the implantable medical device comprising: a posture sensing circuit configured to detect a change within a recumbent posture of a subject; a physiological signal sensing circuit to sense a physiological signal; a controller circuit, operatively coupled to the physiological signal sensing circuit and the posture sensing circuit, the controller circuit configured to detect a change in the physiological signal that occurs in association with the detected change within the recumbent posture of the subject, wherein the physiological signal sensing circuit includes at least one of a heart rate sensing circuit, a blood pressure sensing circuit, a heart sound sensing circuit, a heart rate variability sensing circuit, and a respiration rate sensing circuit and the change in the physiological signal includes at least one of: a sensed change in heart rate that is less than a normal subject's change in heart rate; a sensed change in blood pressure that is different from a normal subject's change in blood pressure; a sensed change in heart sound energy that is different from a normal subject's change in heart sound energy; a sensed change in heart rate variability that is different from a normal subject's change in heart rate variability; and a sensed change in respiration rate, respectively, and wherein the controller circuit is further configured to generate a response as a function of the detected change in the physiological signal; and a pacing circuit coupled to the controller circuit, wherein the response generated by the controller circuit is to adjust pacing delivered by the pacing circuit.
 4. The system of claim 3, wherein the response generated by the controller circuit is to adjust a pacing location of the pacing delivered by the pacing circuit.
 5. The system of claim 3, wherein the response generated by the controller circuit is to adjust a cardiac resynchronization therapy timing between paces delivered by the pacing circuit.
 6. A system comprising: an implantable medical device, the implantable medical device comprising: a posture sensing circuit configured to detect a change within a recumbent posture of a subject; a physiological signal sensing circuit to sense a physiological signal; a controller circuit, operatively coupled to the physiological signal sensing circuit and the posture sensing circuit, the controller circuit configured to detect a change in the physiological signal that occurs in association with a the detected change within a the recumbent posture of a the subject, wherein the physiological signal sensing circuit includes at least one of a heart rate sensing circuit, a blood pressure sensing circuit, a heart sound sensing circuit, a heart rate variability sensing circuit, and a respiration rate sensing circuit, and wherein the change in the physiological signal includes at least one of: a sensed change in heart rate that is less than a normal subject's change in heart rate; a sensed change in blood pressure that is different from a normal subject's change in blood pressure; a sensed change in heart sound energy that is different from a normal subject's change in heart sound energy; a sensed change in heart rate variability that is different from a normal subject's change in heart rate variability; and a sensed change in respiration rate, respectively, and wherein the controller circuit is further configured to generate a response as a function of the detected change in the physiological signal; and a neural stimulation circuit operatively coupled to the controller circuit, wherein the response generated by the controller circuit is to adjust a neural stimulation pulse.
 7. A system comprising: an implantable medical device, the implantable medical device comprising: a posture sensing circuit configured to detect a change within a recumbent posture of a subject; a physiological signal sensing circuit to sense a physiological signal; a controller circuit, operatively coupled to the physiological signal sensing circuit and the posture sensing circuit, the controller circuit configured to detect a change in the physiological signal that occurs in association with the detected change within the recumbent posture of the subject, wherein the physiological signal sensing circuit includes at least one of a heart rate sensing circuit, a blood pressure sensing circuit, a heart sound sensing circuit, a heart rate variability sensing circuit, and a respiration rate sensing circuit, and the change in the physiological signal includes at least one of: a sensed change in heart rate that is less than a normal subject's change in heart rate; a sensed change in blood pressure that is different from a normal subject's change in blood pressure; a sensed change in heart sound energy that is different from a normal subject's change in heart sound energy; a sensed change in heart rate variability that is different from a normal subject's change in heart rate variability; and a sensed change in respiration rate, respectively, and wherein the controller circuit is further configured to generate a response as a function of the detected change in the physiological signal; and a drug delivery control circuit operatively coupled to the controller circuit, wherein the response generated by the controller circuit is to adjust delivery of a substance.
 8. A system comprising: an implantable medical device, the implantable medical device comprising: a posture sensing circuit configured to detect a change within a recumbent posture of a subject; a physiological signal sensing circuit to sense a physiological signal, wherein the physiological sensing circuit is a heart rate sensing circuit; and a controller circuit, operatively coupled to the physiological signal sensing circuit and the posture sensing circuit, the controller circuit configured to detect a change in the physiological signal that occurs in association with the detected change within the recumbent posture of the subject, and wherein the controller circuit is further configured to generate a response as a function of the detected change in the physiological signal.
 9. The system of claim 8, in which the change in the physiologic signal includes a sensed change in heart rate that is different from a normal subject's change in heart rate.
 10. A system comprising: an implantable medical device, the implantable medical device comprising: a posture sensing circuit configured to detect a change within a recumbent posture of a subject; a physiological signal sensing circuit to sense a physiological signal wherein the physiological sensing circuit is a heart sound sensing circui; and a controller circuit, operatively coupled to the physiological signal sensing circuit and the posture sensing circuit, the controller circuit configured to detect a change in the physiological signal that occurs in association with the detected change within the recumbent posture of the subject, wherein the controller circuit is further configured to generate a response as a function of the detected change in the physiological signal.
 11. The system of claim 10, in which the change in the physiologic signal includes a sensed a change in heart sound energy that is different from a normal subject's change in heart sound energy.
 12. A system comprising: an implantable medical device, the implantable medical device comprising: a posture sensing circuit configured to detect a change within a recumbent posture of a subject; a physiological signal sensing circuit to sense a physiological signal in which the physiological sensing circuit is a respiration sensing circuit; and a controller circuit, operatively coupled to the physiological signal sensing circuit and the posture sensing circuit, the controller circuit configured to detect a change in the physiological signal that occurs in association with the detected change within the recumbent posture of the subject, wherein the change in the physiological signal includes a sensed change in respiration rate, and wherein the controller circuit is further configured to generate a response as a function of the detected change in the physiological signal.
 13. The system of claim 12, in which the respiration sensing circuit senses a change in respiration rate when the detected change within recumbent posture includes a change from a first posture to a second posture, wherein the first and second postures are included among a supine posture, a prone posture, a right lateral decubitus posture, and a left lateral decubitus posture.
 14. A system comprising: an implantable medical device, the implantable medical device comprising: a posture sensing circuit configured to detect a change within a recumbent posture of a subject; a physiological signal sensing circuit to sense a physiological signal; a controller circuit, operatively coupled to the physiological signal sensing circuit and the posture sensing circuit, the controller circuit configured to detect a change in the physiological signal that occurs in association with the detected change within the recumbent posture of the subject, wherein the physiological signal sensing circuit includes at least one of a heart rate sensing circuit, a blood pressure sensing circuit, a heart sound sensing circuit, a heart rate variability sensing circuit, and a respiration rate sensing circuit, and the change in the physiological signal includes at least one of: a sensed change in heart rate that is less than a normal subject's change in heart rate; a sensed change in blood pressure that is different from a normal subject's change in blood pressure; a sensed change in heart sound energy that is different from a normal subject's change in heart sound energy; a sensed change in heart rate variability that is different from a normal subject's change in heart rate variability; and a sensed change in respiration rate, respectively, and wherein the controller circuit is further configured to generate a response as a function of the detected change in the physiological signal; and a telemetry circuit operatively coupled to the controller circuit, wherein the response generated by the controller circuit comprises an alert issued by the telemetry circuit to an external device.
 15. The system of claim 2, in which the physiological sensing circuit is a blood pressure sensing circuit.
 16. The system of claim 15, in which the blood pressure sensing circuit senses a change in blood pressure that is different from a normal subject's change in blood pressure.
 17. The system of claim 1, A system comprising: an implantable medical device, the implantable medical device comprising: a posture sensing circuit configured to detect a change within a recumbent posture of a subject; a physiological signal sensing circuit to sense a physiological signal, wherein the physiological sensing circuit is a heart rate variability sensing circuit; and a controller circuit, operatively coupled to the physiological signal sensing circuit and the posture sensing circuit, the controller circuit configured to detect a change in the physiological signal that occurs in association with the detected change within the recumbent posture of the subject, wherein the controller circuit is further configured to generate a response as a function of the detected change in the physiological signal.
 18. The system of claim 17, in which the change in the physiologic signal includes a sensed a change in heart rate variability that is different from a normal subject's change in heart rate variability.
 19. A system comprising: an implantable medical device, the implantable medical device comprising: a posture sensing circuit configured to detect a change within a recumbent posture of a subject; a physiological signal sensing circuit to sense a physiological signal; a controller circuit, operatively coupled to the physiological signal sensing circuit and the posture sensing circuit, the controller circuit configured to detect a change in the physiological signal that occurs in association with the detected change within the recumbent posture of the subject, the controller circuit further configured to identify a health condition as a function of the change in the physiological signal, wherein the physiological signal sensing circuit includes at least one of a heart rate sensing circuit, a blood pressure sensing circuit, a heart sound sensing circuit, a heart rate variability sensing circuit, and a respiration rate sensing circuit, and wherein the health condition is identified by at least one of: a sensed change in heart rate that is less than a normal subject's change in heart rate; a sensed change in blood pressure that is different from a normal subject's change in blood pressure; a sensed change in heart sound energy that is different from a normal subject's change in heart sound energy; a sensed change in heart rate variability that is different from a normal subject's change in heart rate variability; and a sensed change in respiration rate, respectively, and wherein the detected change in posture includes a change from a first recumbent posture to a second recumbent posture, wherein the first and second recumbent postures are included among a supine posture, a prone posture, a right lateral decubitus posture, and a left lateral decubitus posture; and a circuit to generate a response when the change in the physiological signal indicates a change in the health condition; and wherein the circuit to generate the response comprises one or more of a pacing circuit, a neural stimulation circuit, and a pharmaceutical delivery circuit. 